Validation of the Radiello® Diffusive Sampler for the Measurement of Formic and Acetic Acids in Museum Environments

Introduction

The assessment of potential damages to indoor cultural heritage, due to some airborne pollutants, is a major and growing concern for curators and conservators. For years, objects of art in museums have been placed in showcases.

Besides security reasons, this is mainly done to protect these objects against contamination by dust particles and deterioration by gaseous pollutants, a field of growing interest for conservators. In recent years, formic acid and acetic acid were also assumed to be harmful for several objects of art, containing e.g. copper or lead. Measurements inside museum showcases via SPME technique showed that these acids can accumulate and reach high concentrations. Most of the showcases are made of wood and/or composite materials such as MDF (Medium Density Fiberboard), which can be considered a source of low molecular weight volatile organic compounds (VOCs).

This research is especially focused on formic acid and acetic acid. The aim was the development of a monitoring technique based on diffusive sampling and its validation in environmental conditions typical of museum showcases.

The Measurement

Measurements inside microenvironments such as the museum showcases can easily be done by means of diffusive samplers, which require little space and no electrical power. The Radiello samplers provide validated methods for the diffusive sampling and analysis of various gaseous pollutants. To evaluate and validate the applicability of the diffusive sampling of gaseous acetic and formic acids, a radiello diffusive sampler, containing chemi-adsorbing cartridges (RAD166) impregnated with triethanolamine (TEA) in a blue diffusive body (RAD1201) was tested. The quantification was made by ion chromatography.

Analysis

The analytical procedure involves recovery of the acids from the chemi-adsorbing cartridges by means of water extraction (5 mL) followed by two 1 minute mechanical stirring steps, between which the extracts are left to settle one hour. The analysis of aqueous extracts is performed using ion chromatography, quantifying the corres ponding ions (formate and acetate) by means of a conductivity detector with a suppressor module. A peak resolution acetate/formate of 1.95 is achieved.

Exposure Chamber Tests

Two exposure experiments were performed in order to determine the sampling rates. The temperature and air humidity were kept constant at 25 °C and 47% Relative Humidity, respectively. The two experiments were carried out by applying two different levels of concentration:

Low level: 128 μg/m3 of formic acid, 160 μg/m3 of acetic acid

High level: 1248 μg/m3 of formic acid, 1564 μg/m3 of acetic acid

During each experiment, four groups of samplers (6 replicates each) were exposed to the test concentration for variable periods: 24 hours, 2 days, 4 days and 7 days.

The test atmospheres were generated by means of capillary injection of an aqueous mixture of the two organic acids in a nitrogen stream, followed by a further gaseous dilution.

Results

A series of six unexposed samplers were analyzed in order to assess the blank levels (Table 1) and hence the detection limits of the measurement method, defined as three times the standard devi ations. For each compound and concentration level, a set of sampling rate values was calculated. Values in Tables 1 and 2 represent the average and standard deviation of six individual values obtained for a specified exposure time.

Table 1. Blank Value

Table 2. Sampling Rate Value Comparison for Different Exposure Times

The analysis of variance (ANOVA) with two-level factors (concentration/time of exposure) shows that formic acid sampling rate is significantly affected by both concentration and exposure time. On the other hand, acetic acid sampling rate shows a weaker depend ence on exposure time, but no statistically significant dependency on concentration.

Therefore, several sampling rate values must be used for 7-day formic acid sampling. However, the average acetic acid sampling rate, for the same exposure times was determined as 97.3 ± 3.1 mL/min.

The formic acid sampling rates show an unexpected behavior, especially at low concentration. In fact, taking into account the higher value of the diffusion coefficient of that compound, higher sampling rate values, compared to acetic acid, would have been expected.

Conclusions and Perspectives

The measurement method allows the determination of concentrations above 1,000 μg/m3 of both formic acid and acetic acid inside museum showcases, with detection limits below 0.5 μg/m3. The method will be a powerful tool in the assessment of concentrations of low molecular weight organic acids inside museum showcases, as well as for establishing “threshold values” for the protection of art objects.

Future developments will involve the evaluation of other para meters which could influence the sampling rates in typical museum conditions, such as air speed, which is likely to be slow.

In addition, the behavior of formic acid must be investigated further in order to explain the observed deviations from the typical “Fick’s Law” behavior of the sampler.